A fuel cell system capable of small-scale electric power generation at high efficiency has been developed as an electric power generating system of the distributed type capable of implementing high energy-use efficiency because it is easy to construct a system for making utilization of heat energy produced when electric power is generated.
A fuel cell system includes, as a main body of its electric power generating section, a fuel cell. As such a fuel cell, for example, a phosphoric acid fuel cell, a molten carbonate fuel cell, an alkali aqueous solution fuel cell, a polymer electrolyte fuel cell, or other like fuel cell is employed. Of these types of fuel cells, the polymer electrolyte fuel cell is characterized by its ability to generate electric power at relatively low temperatures (from about 50 to about 120 degrees Centigrade) and by its high output density and long service life. Therefore, the polymer electrolyte fuel cell is expected to be applied to electric vehicle's motive electric power sources which are required to have a high output characteristic and a quick startup, to cogeneration systems for household use which are required to have a long-term reliability, or to other like system.
In a polymer electrolyte fuel cell, during its operation of generating electric power, hydrogen-containing fuel gas is supplied towards an anode while on the other hand oxygen-containing oxidizing gas is supplied towards a cathode. As a result, in the anode of the polymer electrolyte fuel cell, hydrogen supplied is converted into electrons and protons. The electrons generated in the anode will reach the cathode of the polymer electrolyte fuel cell by way of an external load connected to the fuel cell system. On the other hand, the protons generated in the anode will reach the cathode through a polymer electrolyte membrane. Meanwhile, in the cathode of the polymer electrolyte fuel cell, water is generated from incoming electrons arriving by way of the external load, incoming protons arriving through the polymer electrolyte membrane, and oxygen present in the oxidizing gas supplied to the cathode. By such a series of mechanisms, electric power is outputted from the polymer electrolyte fuel cell, whereby the external load is driven. Furthermore, fuel gas is supplied from a fuel gas supplier which generates hydrogen, for example, from methane by a steam reforming reaction. In addition, oxidizing gas is supplied from an oxidizing gas supplier which takes in air from the atmosphere, for example, by means of a “sirocco” fan.
Incidentally, in a fuel cell system equipped with a polymer electrolyte fuel cell, it is required that its polymer electrolyte membrane be maintained in a wet condition in order to ensure proton conductivity from the anode side to the cathode side. To this end, in the fuel cell system, fuel gas humidified and oxidizing gas humidified are supplied to the anode side and to the cathode side, respectively. In addition, in the fuel cell system, in order to ensure high energy conversion efficiency when converting a change in the free energy of a chemical reaction into electric energy, the polymer electrolyte fuel cell is operated, for example, in an operating condition that meets the following interrelations: Tcell>Tda and Tcell>Tdc (hereinafter, such an operating condition is referred to as a “low-humidification operating condition”) where Tda is the fuel gas dew point, Tdc is the oxidizing gas dew point, and Tcell is the polymer electrolyte fuel cell temperature. Hereby, the fuel cell system will stably exhibit its predefined electric power generation performance over long periods (for example, see Patent Document 1).
On the other hand, the electric power generating operation of the fuel cell system generally employs such a startup/shutdown type operating mode that the electric power generating operation of the polymer electrolyte fuel cell is started up or shut down depending on the situation, because there is no need to carry out an operation of producing electric power if neither electric energy nor heat energy that the fuel cell system generates is needed. In this startup/shutdown type operating mode, when neither electric energy nor heat energy is required the controller of the fuel cell system first controls the fuel gas supplier and the oxidizing gas supplier to stop their operation and then disconnects the electric connection between the polymer electrolyte fuel cell and the external load. Hereby, the polymer electrolyte fuel cell enters the open circuit state. And in order to prevent the polymer electrolyte membrane from becoming dried, the controller causes humidified inactive gas to be sealed within the inside of the polymer electrolyte fuel cell. In addition, the controller shuts off the connection of the polymer electrolyte fuel cell with the fuel gas supplier and the oxidizing gas supplier, thereby closing off a fuel gas passage and an oxidizing gas passage in the polymer electrolyte fuel cell. Hereby, the fuel cell system prevents the polymer electrolyte membrane from becoming dried over long periods (for example, see Patent Documents 2, 3, and 4).    Patent Document 1: JP-A-H04-502749    Patent Document 2: JP-A-H06-251788    Patent Document 3: JP-A-2004-163037    Patent Document 4: JP-A-2004-006166